In recent years, a cloud service such as IaaS (Infrastructure as a Service) has been spread all over the world. In the cloud service, a service business provider provides a computer system which constructs virtualization environment and users build various services by using virtualization machines allocated in the computer system. As an example, ETSI (European Telecommunications Standards Institute) which is a standardization group has discussed a concept, NFV (Network Function Virtualization), and the standardization thereof is gradually spreading. NFV is a method which provides services which communication businesses have provided by using exclusive devices, by virtual machines. In such background, technologies which allocate the virtual machines to use the computer system effectively have been desired.
As an example of such technologies, PTL 1 discloses the device which is capable of re-allocating a virtual machine based on a design policy in a virtualization system in which virtual servers are built by a plurality of physical severs. There is an example of the design policy, that is, “since a virtual server 2 is a standby system server for a virtual server 1, these are not allocated to a same physical server.” There is another example of the design policy, “since load of works which is processed by a virtual server 3 and a virtual server 4 is small, these may be allocated in the same physical server.”
PTL 2 discloses the virtualization system which continues system operation without reducing the total amount of resource pool when maintenance of a physical machine is carried out, or when a fault occurs in a physical machine. The virtualization system sets up one or more resource pools in which one or more physical machines build (a) virtual machine(s), and a recovery pool which is one shared among resource pools. The virtualization system uses the recovery pool when the physical machine is maintained or when a fault occurs in the physical machine.